WASHINGTON -- Rep. Darrell Issa (R-Calif.) has begun to assert veto control over how Democrats on the House Oversight and Government Reform Committee can travel with their staff, escalating tensions on an already fractious panel.

The latest dispute centers around a request by Rep. Danny Davis (D-Ill.) to take two Democratic committee staffers with him to a drug control policy forum in Chicago on Monday evening. Davis is the ranking member of the Subcommittee on Health Care, District of Columbia, Census and the National Archives, which has jurisdiction over drug control policy.

The committee chairman needs sign off on reimbursements for staff travel, but normally, such requests are routinely approved. Issa, who serves as chair, denied Davis' request.

On Thursday, Oversight Democrats sent a letter to Issa, objecting to his decision and raising concerns about the precedent he was setting.

"Your new policy appears to be that travel by Democratic staff is not for 'official Committee business' unless a Republican joins them," they wrote. "This interpretation is unprecedented and contradicts the Committee’s policy when you were in the Minority. According to Committee records, in December 2010 -- when you were still the Ranking Member of the Committee -- you were accompanied by four minority staffers on a trip to attend unidentified '[m]eetings with technology companies.' You were reimbursed thousands of dollars even though no majority Committee Members or staff joined you."

Jeffrey Solsby, a spokesman for the majority staff on the committee, said Republicans determined that Davis' forum was a personal, not committee, matter.

"The committee didn't feel it was appropriate to allow committee resources to be spent on non-committee business," he said.

Issa also sent Davis a letter offering to schedule a field hearing in Chicago "in the coming weeks" on "national and local initiatives to prevent and treat drug addiction."

But Democrats are arguing that a field hearing shouldn't preclude them from using their own travel budget to send staffers to the forum.

Gregory Abbott, a spokesman for the minority staff of the House Administration Committee, explained that the ranking member is supposed to control one-third of overall committee budgets.

"The 'one-third rule,' which grants budgetary and staffing autonomy to the Minority staff of each Standing Committee, has been a long standing practice in the House of Representatives," Abbott wrote in an email. "In the 112th Congress, every Chair and Ranking Member agreed to uphold and continue that arrangement in a hearing before the Committee on House Administration."

Democrats on the Oversight Committee are arguing that as ranking member and now as chair, Issa has had leeway to bring staff on a wide variety of trips, which were not held up to the same standards as the Davis forum is being subjected to.

For example, when Issa was about to take over as chairman, a committee staffer accompanied him to an electronics convention in Las Vegas, where his own company -- Directed Electronics Inc. -- was promoting its product, the "Viper SmartStart."

"[A] national news reporter was traveling to meet with Mr. Issa at the event and interview him as ranking member and incoming Chairman, and report on him during that visit," Solsby said. "If the reporter had not requested to travel to meet Mr. Issa at that venue, the staff member would not have made the trip."

In April, Issa went back to his home county for a "forum" on biotechnology, which was also promoting the work of the committee's website americanjobcreators.com. Democratic and Republican staffers went and were reimbursed for their travel.

Solsby said Democratic members were invited to attend, but chose not to. Democrats also wrote in their letter to Issa that Davis invited all committee members to attend his forum, but they all declined.

In December 2010 -- right before Issa officially took over as chairman -- he and four committee staffers were reimbursed for "[m]eetings with technology companies," the Democratic letter noted. No Democratic member or Democratic staffer went with them.

Solsby countered that the trip was committee business and included "a day of meetings with prominent technology company executives to assess the impact of the current regulatory climate on their businesses and discuss regulatory reform policies that would help them create jobs."